Modern cable communication networks such as Community Antenna TeleVision (CATV) communication networks, are increasingly bi-directional, allowing subscribers to receive information in the forward direction (television signals, Internet data, telephony signals, etc.) but also to send information into the network in the reverse direction (Internet data, telephony signals, “Pay-per-view” ordering data, etc.). An almost universal structure of such network is the Hybrid Fiber Coaxial (HFC) where optical fiber links are used between the headend or sub-headends (hubs) and some distribution points or nodes where optical-to-electrical and the electrical-to-optical conversion is performed. Coaxial cables are then used for local distribution and connection to the subscribers.
While almost the same signals are distributed in the forward path across the whole network under the head-end control, the reverse path sees a multiplicity of sources funnelling into the network. It is common to group sources from a neighborhood (from hundreds to thousands of subscribers) into a specific optical fiber link to the head-end (optical node to head-end).
The reverse path data reception can be disturbed by many unwanted ingress sources, that can be either associated with abnormal operation of the network or other sources such as external sources (shortwave transmission, CB transmitter, electrical motors, welding machine, etc.) leaking into the cable or subscriber faulty connection (faulty cable modem, poor grounding, household appliances, etc). The problem is multiplied by the funnelling effect of the HFC structure. There is then a necessity to monitor the reverse path to detect abnormal conditions in order to maintain a quality of service.
Reverse path monitoring systems are fairly recent. The most commonly used technique consists of performing spectral analysis using a scanning spectrum analyzer, either analog or partly digital with band-limited FFT. Due to the high cost of scanning spectrum analyzers, a switch is generally used to select one from a plurality of outputs provided on the node receivers for sequential analysis and detection of abnormal conditions by either a single or a limited number of spectrum analyzers. Switch input size varies from 4 to 16 or even a cascade of 16's. The spectral analysis provides particular measurements of parameters such as varying noise floor and specific band emission. While spectral analysis can be very sensitive and allow the detection of small incremental differences with averaging, detection is limited to long duration disturbances due to the time-sharing monitoring sequence between receiver outputs and to the scanning nature of spectral analysis. This method is widely used by many system available in the marketplace, such as the WinMonitor™ (Avantron Technologies), Pathtrack™ (Wavetek), Phasor™ 565 (Cheetah, formerly Superior Electronics), SIMS I and SIMS II (AM Communications), RDU (Cable Resources Inc.), SST (Trilithic), 3010H Hewlett-Packard-Agilent Calan, SAT 330-CTMS21(SAT Corp.) and CIM (Electroline).
In an attempt to improve ingress detection efficiency for CATV systems, a subscriber terminal using local detection processing was proposed by Reichert in U.S. Pat. No. 4,520,508 issued on May 28, 1985, which terminal comprises a signal level measurement receiver being tuned to monitor a possible ingress signal entering the system at selected frequencies within the frequency band of the return paths, and to generate a detected ingress signal accordingly. The terminal further comprises a control signal receiver which receives forward command signals sequentially transmitted by a controller located at the headend of the cable system, specifying the address of the subscriber terminal, the frequencies at which ingress signals is to be monitored and the return signal transmitting frequency. The terminal further comprises a microprocessor receiving the detected ingress signal and the received control signals for controlling the measurement receiver accordingly, and a frequency transmitter connected to the microprocessor and to the cable system to transmit information concerning the detected ingress to the headend. Although such local detection processing approach may improve detection efficiency over known monitoring systems using a centralized processing approach, actual monitoring cycle for each terminal detector is limited by the rate of forward command signals received from the controller located at the headend which sequentially control all the terminal detectors of the system. Moreover, the cost of such distributed system may be prohibitive since each terminal must be provided with particular electronic hardware.
Another known method uses sampling for signal analysis, which can be performed with a band-limited analyzer such as a 1.5 MHz band as provided by Cheetah's DSP-565, or with may any other spectrum analyzer used in zero scan mode. With the DSP-565, FFT is performed on the samples for finer frequency resolution. A similar spectral window approach is taught by Schmidt et al. in U.S. Pat. No. 5,939,887 issued on Aug. 17, 1999, in which data representative of a cable spectral energy level is acquired over a selected frequency window and is then compared against a threshold value corresponding to the minimum energy level of a TDMA carrier signal. Whenever the spectral energy level is found to be lower than the threshold, a display is generated characterizing the ingress over the window. Such data pre-processing approach obviates the difficulty of discriminating an ingress signal during periods of active carrier signal transmission, by triggering the measurement only when a carrier signal interruption is detected. However, when the transmitting data flow is approaching the maximum transmission capacity of the reverse path, the probability to detect an ingress during an inactive transmission period decreases, reducing the reliability of monitoring accordingly.
Another approach used by the Hotzman Engineering system consists of sampling over the whole bandwidth of the return path for analysis in time and frequency domains. The use of a sampling oscilloscope with data transfer under IEEE488 to a PC for software analysis slows down the time response of the system.
In all of the foregoing approaches, actual monitoring of the return path is not continuous due to the time sharing between receiver outputs (through switches), between local detectors (distributed system) or between frequency bands (spectrum analyzer scan), or due to data pre-processing, transfer and analysis. High equipment costs preclude providing each receiver output with an analyzer for continuous monitoring.